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GPS Unit Trail Maps in Error

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GPS Unit Trail Maps in Error

Postby Hetchy » Fri Feb 13, 2009 8:54 pm

Has anyone else noticed the inaccuracy of these gps devices. I refer to the map databases therein, not, of course, the GPS coordinate reciever itself.
While hiking along the PCT in the vicinity of The Tilden Lake trail and Falls Creek in Jack Main(Jackmans 1904) Canyon I encountered a fellow who was lost.. or so he thought. He said he had come down the PCT(nobo) from Bailey ridge to Wilma(Wilmer pre 1960) lake and lost the trail. He then climbed back up(ugh!) Bailey ridge and decended via the Tilden Canyon & lake route back to the PCT where I met him.
And after consulting his GPS I saw clearly the database map was very much in error. The base map depicted the Jack Main trail crisscrossing Falls creek for a mile in the vicinity of Wilma lake. Being a map and compass guy it never occured to me the trail could be misplaced in this manner.. must be very frustrating judging by his indignant reaction. Indeed there was no snow obscuring the trail and not to make light of the guy. But turning back on a trail because an electronic box tells you it does not exist?
Let me be clear: I own a GPS but I have found it's usefullness is minimal and even dangerous in a similar way that sitting too close to the fire can blind oneself to the outer world. Time spent waiting for the answers from space and staring at a little box is less time spent reading the land and self routing. I am not saying they are not useful, or that they are evil. I am trying to put my finger on the possible emergence of a trend that has obvious benefits("user friendly" compared to paper maps) but possibly understated negative consequences (not knowing how to get along without it).. especially to people newer to the backcountry scene.
Rather than trust the technology to show us something, Is it not better to learn to navigate from a compass and fixed map that require no batteries or signal from space.. just the signal from your brain to your feet? To treat the GPS as icing on the cake so to speak... secondary to the paper map and compass and BOTH are secondary to our personal judgement.
Maybe I am way out there on this one but I also have never run into someone who totally trusted the GPS, even over his own judgement, until that trip.
Anyone else seen this phenomenon?
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Re: GPS Unit Trail Maps in Error

Postby oldranger » Fri Feb 13, 2009 10:04 pm

yep, My son in law! A smokejumper. On the few occasions he has to hike out from a fire he has jumped he totally has to rely on the gps to determine which way to travel to get out. On a sunny day at noon he can't figure out which way is north! Other than that defect he is a good son in law.

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Re: GPS Unit Trail Maps in Error

Postby rlown » Sat Feb 14, 2009 1:42 pm

Hetchy wrote:Has anyone else noticed the inaccuracy of these gps devices. I refer to the map databases therein, not, of course, the GPS coordinate reciever itself.


I've seen this with Garmin's MapSource. I always project my routes onto Google Earth to get a feel for how accurate my path might be, and it's been off.

Again, I've also experienced situations where the GPS was right and other's "judgement" about where we should go (mainly off-trail) was wrong. It's almost always best to designate someone on trips involving more than 4, to be "trail boss" if you will. Opinions about where one is get's in the way.

I carry a map and compass as a backup as well, because electronics can't always be trusted in the backcountry (battery failure, electronics failure, satellite collisions in space, etc..)

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Re: GPS Unit Trail Maps in Error

Postby gdurkee » Sat Feb 14, 2009 4:14 pm

Map est non tractus: the map is not the territory.

No matter what you're using, whether map, gps or compass, you've got a lot of error potentially creeping into navigation. First is the map itself, of course. Trails are always going to be among the worst, because they're rarely directly surveyed -- using a theodolite and going point to point. The High Sierra Trail is actually surveyed that way, so it's pretty well represented on a map. But most other maps are drawn in by either 'best guess" and some ground truthing of points or, for the last few decades, going from aerial or satellite images and tracing them onto a map. It's darned hard to do.

I can think of several trails showing as crisscrossing a stream and don't. Probably some sort of data mismatch when one is projected onto the other.

That's kind of the problem with GPS guy that you saw. Someone expected everything to match up perfectly and his brain crashed when it didn't. With a map and compass, you don't get that sort of accuracy so you don't notice things like that. You have to pay attention to the terrain.

And, actually, I've found the GPS itself to sometimes be off depending on the time of day (we're only talking about 20 feet here, so it's not critical to finding your way around). At McClure two years ago, myself and another researcher were getting bad location information in the late afternoon on two separate gizmos.

We've just started doing tracklogs of all backcountry trails using hand held GPS units. When I project them, they're often off by 10 feet to even 200 feet of the USGS maps. But the error is consistent every time I go over the same trail, so it's obviously the map.

Finally, there's the whole bunch of errors introduced by how that map is represented on the earth -- how it's "draped" over the terrain. There's different methods for different parts of the earth. I don't know how Google does it on a world scale to give consistently accurate information in front of your feet -- maybe Eric knows and can jump in.

So far, though, I don't see much sign of people so focused on their gizmo that they don't watch the terrain. And, off hand, don't know of any people getting into trouble because they trusted the GPS over common sense. On the other hand, I've navigated in two whiteouts (< 15 foot visibility) with a GPS and it worked great to get me back to the cabin.

That said, I'm pretty darned impressed with my GPS (regular USGS map on a PDA with a GPS -- a lot of acronyms increases the accuracy). I rarely use a compass anymore except to get an angle on something. Down in a flat and horizonless forest, I'd probably be stumbling around lost with Mike's in-law. At least we'd have MRE's and a chain saw.

g.
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Re: GPS Unit Trail Maps in Error

Postby Hetchy » Sat Feb 14, 2009 9:17 pm

I am glad y'all took it that way, in the way it was meant, cause I did not mean to hate on the GPS or those who use it..
Indeed I am a user of a GPS. I just thought it was worth conversation, at least regarding it's limitations.
And as you have highlighted the actual limitations of paper maps themselves.
I get lost, even with the map and compass.. and the GPS thingy as well.. snow has a peculiar way of disguising the familiar and creating new places where none existed before. Perhaps the marriage of GPS and paper maps and hiker reconnaisance will enhance a solid form of ... something else.
In the far future I can see a Google Earth /GPS/CDEC snow survey/Paper map/hiker GPS reconaisance GPS.. Hybrid in digital form. OMG I am scaring myself with this crap! Why did I give up Beer?
Anyway.. My only intent in this topic was to question the: "rungs of the ladder" that are skipped when climbing to the top of the trail. And further if at some future point those skipped "rungs" might prove vital to those in the backcountry caught unaware by a possibly inoperative GPS unit.
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Re: GPS Unit Trail Maps in Error

Postby fishmonger » Sat Feb 28, 2009 9:27 am

one thing I noticed last summer on the JMT is that either the GPS is nuts, or just about all map distances on what's out there on 24k and other popular maps are underestimating the actual distance.

Even with the 60csx at battery saver mode (which straightens out the track you are on quite a bit, reducing recorded hiking distance on twisty trails), we recorded between 20 and 33% more distance on just about any segment of trail. Based on that, I think the Muir Trail is actually quite a bit longer than officially posted, or, my GPS is junk (which it isn't on any paved road, as it matches distances posted where people can measure with a vehicle spot on)

This summer we will do a JMT yo-yo hike and I should be able to get every inch of the official trail recorded, even though we plan on some alternate routes, yet with both directions combined we should have data for the entire official trail. My guess is that it'll be around 235 miles

I bring maps for orientation and planning, the GPS mostly for tracking progress .
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Re: GPS Unit Trail Maps in Error

Postby gary c. » Sat Feb 28, 2009 3:10 pm

fishmonger, you need to remember that when you download a trail to your GPS unit you are only downloading a series of points along the trail and the GPS only measures the distance in a straight line from point to point. In between those points the trail trail curves, goes up and down and just about anywhere until the next actual downloaded point from your software. When you actually track the trail as you walk the GPS unit is recording your progress by recording logged position points at a much more frequent interval so it takes out more of the curves. Even then the GPS unit is stil only mearureing in a straight line from point to point.
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Re: GPS Unit Trail Maps in Error

Postby gdurkee » Sat Feb 28, 2009 4:15 pm

I just found a discussion on another board on how slope is represented by a GPS unit. The answer, as I've always kinda suspected, is it isn't:

"I have talked with the rep at Garmin and he states that: There are NO GARMIN products, at the present time, that can give ACCURATE distances with any significant changes in elevation. He gave as an example, in a 1 mile distance with a 10 degree change in elevation, there will be a shortfall of 150 feet. GARMIN products are only a 2 dimensional device (with respect measuring distance).... The best way to get an accurate measurement in distance is the use of a mileage wheel."

Note: applies to all makes not just Garmin.


I'm not sure where the 212 (or whatever) mileage came from in the JMT mileage. That's the distance to the top of Whitney as shown on a sign at Happy Isles -- that's a very old sign, from the 1950s at least (hmmm, wait. Strike "very" and substitute "venerable" since I suspect the sign and I are the same age...), but I don't know what it's derived from. It would be interesting to find out. Wheeled is the only accurate way to do it. Most of the trails in Sequoia Kings have been wheeled but, here's the fun part, when it comes time to make a new sign, they rarely look up the wheeled distances that are stuffed in a file drawer somewhere and just use what the sign says.

That same forum noted that when the GPS loses a signal or gets an inaccurate one -- which happens all the time -- you get a random or only semi-accurate position read, when you get the right one, the total distance of all that is calculated in with your reading.

Here's the discussion:

http://www.exploroz.com/Forum/Topic/53040/GPS_vs_Speedo.aspx?ky=%22Gps%22&p=%2FForum%2FDefault.aspx%3Fs%3D1%26ky%3D%2522Gps%2522%26pn%3D1

We're now in the process of doing GPS tracklogs of a lot of the trails that may not have been accurately drawn from Satellite photos, but don't know what that does to distance measurements.


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Re: GPS Unit Trail Maps in Error

Postby oldranger » Sun Mar 01, 2009 3:39 pm

The only time I use my gps is when doing my training hikes here in Central Oregon. One of my favorites is the trail up Tumalo Mt. It is a little less than 2 miles and 1400 vertical feet. My gps is one designed for runners so I wear it on my wrist. It is .1 mile further if I wear the gps on my right wrist than on my left. I have suspected that is because the right wrist is predominantly on the south where more satellites are located so there are fewer lost signals and less of the averaging that George talked about. I never thought about effects of elevation gain and loss. I knew it had to be more than 1.9 miles to the top!

Time to get my dose of outside for the day.

mike
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Re: GPS Unit Trail Maps in Error

Postby fishmonger » Wed Mar 04, 2009 12:33 pm

gary c. wrote:fishmonger, you need to remember that when you download a trail to your GPS unit you are only downloading a series of points along the trail and the GPS only measures the distance in a straight line from point to point. In between those points the trail trail curves, goes up and down and just about anywhere until the next actual downloaded point from your software. When you actually track the trail as you walk the GPS unit is recording your progress by recording logged position points at a much more frequent interval so it takes out more of the curves. Even then the GPS unit is stil only mearureing in a straight line from point to point.
Gary C.


I wasn't downloading trails - I was recording them, then comparing those distances to what maps state. Meaning, my straight line zig zag recording that should be shorter was still 20-30% longer than what the topo maps state.
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